1. Technical Field
The present invention relates an X-ray fluorescence analysis apparatus that is provided with a radiation detector configured by a superconducting transition edge sensor.
2. Description of the Related Art
As an X-ray fluorescence analysis apparatus that is capable of discriminating energy of an X-ray, there are an energy dispersion-type X-ray detector (energy dispersive spectroscopy, hereinafter referred to as EDS) and a wavelength dispersive spectroscopy, hereinafter, referred to as WDS.
The aforementioned EDS is an X-ray detector of a type in which energy of an X-ray taken in a detector is converted into an electrical signal in the detector and the energy is calculated according to the size of the electrical signal. The WDS is an X-ray detector of a type in which an X-ray is subjected to monochromatic processing (discrimination of energy) by a spectroscope and the monochromatic X-ray is detected by a proportional counter tube and the like.
As the EDS, there are known semiconductor detectors such as an SiLi (silicon lithium) detector, a silicon drift detector, and a germanium detector. For example, the silicon lithium detector and the silicon drift detector are frequently used in an element analyzer of an electron microscope and can detect energy in a wide range of approximately 0.2 keV to 20 keV. However, since silicon is used in the detector, the property thereof depends on a band gap (approximately 1.1 eV) of silicon, in principle. Therefore, it is difficult to improve energy resolution to approximately equal to or greater than 130 eV, and the energy resolution of the EDS is ten or more times inferior to that of the WDS when compared.
When energy resolution which is an index indicating a performance of an X-ray detector is 130 eV, for example, it denotes that energy can be detected with an uncertainty of approximately 130 eV when an X-ray detector is irradiated with an X-ray. Accordingly, as the uncertainty decreases, the energy resolution increases. In other words, when detecting a characteristic X-ray consisting of two spectrums adjacent to each other, if the energy resolution increases, the uncertainty decreases. When an energy difference between two peaks adjacent to each other is approximately 20 eV, the two peaks can be separated at the energy resolution of approximately 20 eV to 30 eV, in principle.
Recently, an energy dispersion-type super-conducting X-ray detector having energy resolution equivalent to that of the WDS is attracting attention. A detector having a superconducting transition edge sensor (transition edge sensor, hereinafter, referred to as TES) among the super-conducting X-ray detectors is a highly sensitive calorimeter utilizing a rapid resistance change (for example, a temperature change is several mK, and a resistance change is 0.1Ω) of a thin metallic film during a transition between super-conduction and normal conduction. The TES is also called as a micro calorimeter.
The TES detects temperature changes in the TES occurring when a fluorescent X-ray or a characteristic X-ray generated from a sample by being irradiated with radiation such as a primary X-ray and a primary electron ray is incident, thereby analyzing the sample. The TES has higher energy resolution than other detectors. For example, the TES can acquire energy resolution equal to or less than 10 eV in a characteristic X-ray of 5.9 keV, for example.
When the TES is mounted on a scanning electron microscope or a transmission electron microscope, the TES obtains a characteristic X-ray generated from a sample which is irradiated with an electron ray, and thus, it is possible to easily separate peaks of an energy spectrum of a characteristic X-ray (for example, Si-Kα, W-Mα, and W-Mβ) which cannot be separated by a semiconductor-type X-ray detector.
Since the TES is a highly sensitive calorimeter, multiple sheets of heat shields are necessary to ensure a stable operation. However, since X-rays generated from a sample need to be introduced to the TES, X-ray windows are equipped in the heat shields as described in Non Patent Document 1, which is identified below. In Non Patent Document 1, the X-ray windows are equipped in the heat shields which are individually cooled to 4 K and 80 K. The X-ray window allows an analysis target X-ray to pass through but performs shutting-off against visible light and infrared light which cause noise.
Moreover, apart from the heat shields, in order to make the TES into one vacuum chamber, X-ray windows having vacuum resistant properties are provided so as to perform shielding against the outer atmosphere at room temperature. In general, as the X-ray window having a vacuum resistant property, an X-ray window adopting an organic membrane is utilized as described in Non Patent Document 2, which is identified below. When three sheets of the X-ray windows are equipped, X-ray transmittance thereof greatly drops to equal to or less than 1% (less than 0.2 keV) from 60% (1 keV).
In addition, since the size of an X-ray absorbent provided in the TES is small (several hundred microns), an X-ray lens is provided between a sample and the TES for the purpose of increasing an effective solid angle. A capillary is generally formed with a thin glass tube. This type of configuration is disclosed in: U.S. Pat. No. 5,880,467; JP-B-1995(H07)-040080; JP-B-1995(H07)-011600; and JP-A-2008-039500.
Non Patent Document 1: Keiichi TANAKA et al (nine), “Transition Edge Sensor-Energy Dispersive Spectrometer (TES-EDS) and Its Applications”, “IEICE TRANSACTIONS on Electronics”, vol. E92-C No. 3, 2009, p. 334 to 340
Non Patent Document 2: AP Ultra-thin Polymer X-ray Windows. [online]. MOXTEK Incorporated, 2010. “retrieved on 2013-10-15”. Retrieved from the Internet: <URL: http://moxtek.com/xray-product/ap-windows/>
However, in the X-ray fluorescence analysis apparatus described above, efficiency of X-ray transmittance equal to or less than 1 keV is unfavorable, and for example, detection efficiency of boron (183 eV) is one level lower than the existing silicon drift-type semiconductor detector (silicon drift detector, hereinafter, referred to as SDD). The reason is that the TES requires two or more X-ray windows for heat shields in addition to an X-ray window formed with an organic membrane, compared to one sheet of the X-ray window formed with the organic membrane which is enough for the SDD.
The TES has an operation temperature lower than that of the SDD so that the TES needs to be provided with extra X-ray windows as the heat shields for a stable operation, compared to the SDD. As described above, it is necessary that the TES can be thermally stable and acquire efficiency of X-ray transmittance equivalent to or greater than that of the SDD in order to efficiently obtain an X-ray equal to or less than 1 keV by the TES.